An oscillator is an energy conversion apparatus that converts direct current electric energy into alternating current electric energy having a specific frequency. It is widely applied to multiple fields such as measurement, automatic control, wireless communications, and remote control. In a wireless communications system, an oscillator (for example, a carrier oscillator of a transmitter) is mainly used to generate and output multi-frequency oscillation signals. Operating bandwidth of the wireless communications system depends on a frequency range of the oscillation signals. With development of the wireless communications system, the wireless communications system needs to have larger operating bandwidth, and the oscillator used in the system needs to output oscillation signals having a frequency range as wide as possible. A dual-mode oscillator can break a frequency range limitation of a conventional oscillator (for example, an LC oscillator) and provide oscillation signals having a wider frequency range, and therefore is more applied to existing wireless communications systems.
As shown in FIG. 1, an existing dual-mode oscillator usually includes two LC (inductor-capacitor) oscillators and one mode switching circuit. Each LC oscillator includes a pair of transistors and an LC oscillation circuit. Two switch groups, that is, switches S1 and S2 and switches S3 and S4, in the mode switching circuit are alternately turned on, so that the two LC oscillators are in two different operating modes. When S1 and S2 are turned on and S3 and S4 are turned off, the two LC oscillators are in an in-phase mode, and an oscillation signal frequency range of the dual-mode oscillator is the same as a frequency range of a single LC oscillator. When S1 and S2 are turned off and S3 and S4 are turned on, the two LC oscillators are in a reverse-phase mode, and an oscillation signal frequency range is lower than that in the in-phase state. Because the dual-mode oscillator has a high frequency range and a low frequency range respectively in the two operating modes, compared with the conventional oscillator, the dual-mode oscillator can provide an oscillation signal having a wider frequency range.
Though the existing dual-mode oscillator can provide and output oscillation signals having a wide band, phase noise performance of the existing dual-mode oscillator is relatively poor, which is one of important factors limiting performance of a communications system. Therefore, it is necessary to provide a new oscillator, to ensure good phase noise performance while providing wide-band oscillation signals for output.